There are a variety of applications that securely wipe a disk like shred and dd. dcfldd is an enhanced version of dd with features useful for forensics and security. It accepts most of dd's parameters and includes status output. Install dcfldd from the official repositories.

Note: Fdisk will not work on GPT formatted devices. Use gdisk instead.

Overwrite the disk

Warning: There is no confirmation regarding the sanity of this command so TRIPLE CHECK that the correct drive partition or drive has been targeted!

Zero-fill the disk by writing a zero byte to every addressable location on the disk using the /dev/zero stream. Make certain that the of=... line points to the target drive and not to a system disk! Make sure to specify a blocksize (i.e. bs=xx). The speed of the overall process will be affected if omitting this switch.